This invention relates to alarm circuits and in particular to alarm circuits adapted for use in vehicles. With the advent of the modern automobile at the end of the second World War, the electronic arts have provided for a variety of warning devices adapted to detect malfunctions or dangerous conditions in a vehicle. For example, passenger automobiles have long hand ammeters to detect a weak battery condition, temperature gauges to detect overheating of the cooling system, and oil gauges to detect loss of oil pressure. Some or all of these devices have been electrical, utilizing electric currents and sensing circuits to provide the necessary indications. In the case of trucks, the particular conditions to be monitored may differ, but the end indications are the same: generally a light, gauge or buzzer is actuated to indicate an unsafe condition.
In recent years, due in large part to federal legislation, passenger vehicles have been equipped with seat belt warning devices serving as remainders to use seat belts or, in the case of late models, enforcing use of the seat belts prior to permitting operation of the vehicle. Many of these latter types of alarms have been of the buzzer or tone alarm variety. Such an alarm utilizes a common buzzer to generate an audible tone when the electronic circuit to which it is connected detects the absence of a necessary condition in the vehicle. An electronic interlock may be incorporated whereby if a driver does not follow a prescribed sequence, the starter motor is electronically disconnected from the battery and an alarm is energized, indicating the failure to follow the required procedure. Additionally, if a passenger is seated next to the driver, the system also requires that this passenger buckle up before the alarm will stop.
While the above described warning system has met with some success, there is a great deal of objection to its mandatory aspects. Furthermore, a serious problem has been encountered wherein only one person is inside the vehicle but a package or other heavy object is placed on the seat next to him. The package usually cannot be strapped in, but nevertheless the tone alarm circuit detects its presence and is fooled into believing that a nonexistent person occupies the seat. Accordingly, the alarm circuit is energized resulting in a continuous buzzing in a passenger compartment. This can be distracting and unsafe and has resulted in a number of new car purchasers attempting to completely defeat the entire warning system.
In view of the problems with present alarm systems, certain modifications have been proposed. A first modification would include removal of the mandatory starting procedure as a condition for connecting the starter motor to the battery. Of particular interest to the present invention, however, is a proposal to provide a self-defeating or automatic turnoff alarm system. Such a system would sound an alarm if seat belts are not buckled. However, if the condition continues after a predetermined period, the alarm would cease and reset itself for subsequent use. The vehicle alarm systems presently available do not possess such a capability. It is to be pointed out that vehicle alarm systems typically must run on a 6- or a 12-volt battery system and be capable of withstanding extreme temperature ranges, require little service, and be low in cost.
It is accordingly an object of the present invention to provide a vehicle alarm circuit which shuts off after a preselected period.
It is another object of the present invention to provide an improved vehicle alarm circuit which provides an interrupted tone.
Other objects and advantages of the present invention will become apparent from the concluding portion of the specification.